St. Croix Crossing

Design and construction

Learn more about the design and construction elements of the St. Croix Crossing project. Choose from the following table of contents, and stop back often. We will continually update this information and explain more activities as construction progresses.

Design

Construction

Loop Trail

The Loop Trail is a 4.7-mile bicycle and pedestrian trail that is part of the St. Croix Crossing project. When the new St. Croix Crossing is complete as planned in 2016, vehicle traffic will be re-routed onto the new bridge and the existing Stillwater Lift Bridge will be converted to a bicycle and pedestrian facility. The Loop Trail will cross the St. Croix River at the Stillwater Lift Bridge and on the new St. Croix Crossing bridge. Trails in Minnesota and Wisconsin will complete the loop and will connect to regional and local trail systems. Please note that the Lift Bridge will still need to accommodate boat navigation with lifts.

The Loop Trail consists of a number of segments that will be completed by 2017. The main portion of the Minnesota Loop Trail will be constructed in 2015 and 2016, with final connections made in 2017. Construction of the Wisconsin portion of the project will occur in 2017.

Crossbeam

A crossbeam is a horizontal structure that connects an upstream pier column to a nearby downstream pier column. The new river crossing will be made up of five crossbeams, one at each pier location. The pre-cast concrete segments, which make up the driving surface, frame into each crossbeam and carry all of the bridge superstructure load back to the piers.

Construction process:

Crews install a temporary truss system to support construction efforts

Crews pour wet concrete on site into the forms and over the rebar and tendons

These steps are repeated three times – because of the size and detail of each crossbeam, they are constructed in three stages

Crews remove the forms once the concrete has cured to its desired strength

The steel bars and strands inside the structure are tensioned, or pulled like a rubber band, in both directions of the crossbeam

Quick facts:

Each crossbeam is 18.2 ft. tall, 15.2 ft. wide and 116.4 ft. long

The total rebar in each crossbeam weighs 478,000 lbs.

The total concrete in each crossbeam weighs 5,273,000 lbs. – that’s 130 truckloads!

Pier table

View of the Pier 8 pier table under construction. The pier table is poured on-site in the shape of a segment.

Constructing the pier table is the next linear step in the bridge deck construction process. Think of the pier table like a tabletop. Together with the crossbeam, it forms a large, flat surface above the pier columns. The top of the pier table will become the actual driving surface.

Construction process:

Pier table construction uses a cast-in-place design

Forms are fitted on each side of the crossbeam in the shape of a segment (see photo). Each side is 15 feet wide

Crews pour wet concrete on site into the forms and over the rebar and tendons

This process is repeated several times to construct the bottom, top and side walls of the pier table – the structure has hollow space in its center, similar to the pre-cast segments that are made off-site

Crews remove the forms once the concrete has cured to its desired strength

Driving surface

The driving surface of the new river crossing and its approach ramps will be constructed in two ways, using both pre-cast segments and a cast-in-place approach.

Pre-cast segments

About 330 pre-cast concrete segments made at the on-site casting yard will become the driving surface for the bridge approach spans in Minn.

The new St. Croix Crossing will be made up of about 1,000 pre-cast segments. The segments are produced off-site at two casting yards:

Cast-in-place sections
Portions of the bridge driving surface will be made up of cast-in-place box-shaped sections instead of pre-cast segments (see purple on above map).

Construction process:

Crews install falsework between Piers 5 and 7 that will carry Hwy 36 traffic and ramps onto the new river spans.

First, falsework goes up. Falsework is a series of temporary structures that support the bridge spans during construction until the bridge can support itself. The falsework is not a permanent part of the bridge

Forms are installed in the shape of a very long segment—this length varies depending on the approach span under construction

Stay cables

View of the 76 strands that make up a stay cable.

The new St. Croix Crossing is an extradosed bridge, or a cross between a box-girder and a cable-stayed design. They are located above the bridge’s driving surface. They anchor to the pier tower on one end and a stay segment on the opposite end. Stay cables help support the load, or weight, on the bridge and are necessary to have 600-foot spans between the river piers.

The upper part of the pipe is high density polyethylene and the lower part is stainless steel.

The pipes are assembled on the bridge deck then raised into position.

Crews string each strand through the pipe by hand.

Crews stress--pull tight like a rubber band--each strand after it is installed from inside the bridge using a hydraulic jack that applies 33,000 lbs. of force.

The cables and anchorages are sealed to prevent corrosion

Quick facts:

Eight stay cables will stick out from each side of the pier towers. Each pier location on the river will have 32 total stay cables.

Total length of stay cables: 5.2 miles (about 400 miles of cable strands)

Each cable has a total stressing force of 2.5 million lbs.

Pier 13

Pier 13 foundation pile driving on the Wisc. bluff. Photo from October 2015.

Pier 13 is the single pier location on the Wisconsin bluff. Like the river piers, this pier will consist of two columns to support eastbound and westbound traffic. The contractor is using a top-down construction approach to decrease the impact on the bluff. Construction crews have constructed a temporary trestle to allow them to get crews and necessary equipment out to the Pier 13 work site. Crews are also limited to 10 ft. of tree removal on each side of the bridge to prevent soil erosion and to help the bridge blend into its surrounding environment.

Bridge foundations

All of the bridge foundations/footings below the water surface were completed in 2013. There are two concrete footings at each of the five piers in the water. At the end of 2013, they raised up about 15 ft. above the water level. Today, they are much higher.